The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive using a high molecular weight styrene-ethylene-ethylene-propylene-styrene copolymer (SEEPS) to provide high initial bond resistance for making elastic components such as laminates containing elastic strands for use in disposable diapers.
The increasing complexity of manufactured goods, in particular disposable goods, also leads to major improvements and developments in the hot melt adhesive industry. Hot melt adhesives are being used to bond a wider variety of substrates, within a broader adhesive application process window, and for a large end-use portfolio. For example considering the diaper manufacturing industry, materials involved may be non-woven materials, polymeric films, and in general elastomeric components. These elastomeric components can be used in products like diapers, in a form of strands, films, nonwovens or any other continuous or discrete form.
Processability of hot melt adhesive is linked to their ability to be melted, and transported and/or coated in a molten stage to the final location where the bond is required. Usually the molten adhesive is sprayed, or coated as a film. Once cooled down, the adhesive needs to fulfill multiple requirements, like bond strength measured by peel force or bond retention under or after mechanical stress, and under or after various thermal conditions.
Typically hot melt adhesives can be based on polymers such as polyolefins (ethylene- or propene-based polymers), or functionalized polyolefins (ethylene or propene copolymers with oxygenated function containing monomers), or styrene block copolymers containing at least one rubbery phase, like SIS, or SBS. Styrene block copolymers are of interest due to their dual characteristics, i.e. cohesion of the styrenic phase associated with the rubber behavior of another phase. Typical application temperatures are equal to or higher than 150° C.
Combining parameters in the areas of a substrate's nature, adhesive processability and a product's end use requirements, there has been a steady trend in the industry to change and use more sophisticated substrate types, for technical or economical reasons. This can lead to the use of more sensitive substrate materials, in terms of mechanical, thermal, weather or time resistance, with the need to not compromise any of the other attributes, i.e. the overall manufacturing process should remain of the same concept, and the end use of the item should be fulfilled in the same way, or enhanced. For example in the diaper industry, typical application temperatures for elastic attachment would be around 163° C. Depending on the bonding performances required, however, it may be higher. Lowering the application temperature presents problems in terms of wet-out, and most of the time 150° C. would be seen as a minimum temperature one can go to attach elastic parts onto the diaper structure.
It is known in the diaper industry that the use of heat sensitive substrates may cause problems if the adhesive temperature is too high because the production line has to be stopped each time the substrate breaks or is damaged by the molten adhesive material (described as a “burn through” phenomenon) and would need to be replaced or fixed before starting the line again. This may also be the case with non-woven substrates or with elastomeric components used in the diaper structure. Thus, a lower application temperature of the hot melt adhesive would be very helpful to avoid maintenance issues and downtime on production lines.
Another factor making it desirable to reduce the application temperatures of hot melt adhesives is that the diaper industry has been trying to use thinner gauge films in order to decrease the overall diaper's material weight, and consequently the material cost. Over the years, this has been achieved with more or less success, depending on the difficulty to keep both the manufacturing process and end-use attributes the same. Heat distortion or deterioration of the film or non-woven substrates can occur easily when the hot adhesive material contacts the substrates' surfaces. As a result, the functionality of the substrates in the end-use structure is affected in a way that is not acceptable. Among other reasons to decrease the application temperature of the hot melt adhesives is the concern of saving some energy cost needed to heat the adhesive material, as well as the need to enhance safety for the workers on the production line to minimize potential burn hazards.
Many references offer possible solutions to apply a hot melt adhesive material at low temperature. Lowering material viscosity is very often seen as the only criterion to lower the application temperature. Both lack of cohesion and incompatibility of composition ingredients, however, have hindered solving this problem in the manner in which the present invention proposes to solve the problem.
It has to be noted that the phrase “low application temperature hot melt adhesive” as used herein corresponds to the ability to apply the molten or deformable adhesive material at a relatively low process temperature, or “application temperature”, i.e. less than 150° C., in order to build a bond between two substrates.
Adhesive application at low temperature is relatively easily achieved for specific applications or application domains where there is no stringent cohesion required. Although focus could be put on Shear Adhesion Failure Temperature (SAFT) value, the aim of this test is more in defining a failure under a constantly increasing temperature than reflecting the mechanical resistance of the bond over time. Many references exhibit interesting SAFT values that do not correlate with the ability of the adhesive materials to resist creep conditions over an extended period of more than few minutes at elevated temperature.
Numerous references claim the concept of applying an adhesive at low temperature, with a certain lack of precision, i.e. they do not precisely define the temperature domain, or they do not give a clear way of how to practically achieve the low temperature application.
Aromatic resins, including pure monomer resins, are commonly used raw materials to formulate hot melt adhesives, those being PSA or not. The softening point of these materials is typically between 5° C. and 160° C., and their presence in the formulas can be driven by the level of tack and of adhesion required, as well as by the need to reinforce the styrenic phase of any styrenic block copolymer. Reinforcing resins help to provide a higher cohesion to the adhesive bond, at room temperature as well as at elevated temperature.
Finally, no mention or preference is made for the softening point value of the cited tackifying resins, both mid-block resins and end-block resins. Softening point of such tackifying resins is an essential characteristic of these ingredients in regard to low application temperature and is a key parameter for the present invention. These three last points show that the information disclosed in this reference is unable to teach one skilled in this art anything that would bring any relevance to the present invention.
There are a number of prior art patents that discuss the use of other saturated mid-block styrenic block copolymer for use in hot melt adhesives. The block copolymers include styrene-ethylene/butylene-styrene, styrene-ethylene/propylene-styrene triblock copolymers, and styrene-ethylene/propylene diblock copolymers. These polymers are all different than the styrene-ethylene-ethylene/propylene-styrene block copolymers used as the primary polymer ingredient in the adhesive formulations of the present invention.
U.S. Pat. No. 5,863,977 discloses the use of substantially linear, high molecular weight saturated mid-block polymers to produce hot melt adhesives. The mid-block is further defined as ethylene/butylenes, ethylene/propylene or mixtures thereof. This patent is absent any teaching on any critical aspects (chemistry, molecular weight, softening point, etc.) of the tackifying resin used in the adhesive. Further, it does not contemplate the use of SEEPS block copolymers or an aromatic reinforcing resin.
U.S. Pat. No. 5,912,295 discloses a removeable hot melt pressure sensitive adhesive having 5 to 30% of a block copolymer having styrene endblocks and ethylene/butylenes or ethylene/propylene midblocks and having a diblock content of greater than about 35% and a melt index of greater than about 20 grams/10 minutes.
U.S. Pat. No. 6,197,845 discloses articles using hot melt adhesives for adherence to skin. The adhesives used therein comprises block copolymers having styrenic endblocks and a midblocks comprising ethylene/butylene, ethylene/propylene and mixtures thereof. However, it does not contemplate the use of SEEPS block copolymer or an aromatic reinforcing resin.
U.S. Pat. No. 6,465,557 discloses a hot melt pressure sensitive positioning adhesive for use with an absorbent article. The adhesive comprises from 6 to 15 percent of a hydrogenated styrene-(butadiene and/or isoprene)-styrene block copolymer having a vinyl content of greater than 50%, from 50 to 80 percent of a tackifying resin which has an aromaticity such that the MMAP cloud point is at least 45 degrees Celsius and from 5 to 35 percent plasticizer. No other end uses are contemplated in this patent. The MMAP cloud point restriction in this patent excludes the use of aromatic resins that are useful in the present invention. In addition, the polymers used in the present invention have very low vinyl contents (preferably less than 10 percent).
EP 1700895 A1 discloses a peelable hot melt adhesive for automotive applications. The formulations shown in this patent all comprise blends of SEEPS and SEP diblock polymers in combination with calcium carbonate. The filler content in combination with the high viscosity of the finished adhesive (greater than 15,000 centipoise at 180° C.) would result in hot melts that would not be suited for spray applications of any kind. The adhesives of the present invention are designed to be low viscosity, non-filled, non-peelable, sprayable hot melt adhesives.
Japanese Patent Abstract 2000-219860 issued to Kuraray describes low molecular weight hydrogenated block copolymers that can be used to make pressure sensitive hot melt adhesives. The number average (Mn) molecular weight of the polymers described range from 93,000 to 114,000. These low molecular weights require high polymer loadings to give acceptable strength and viscosity. By contrast, the polymers in the instant invention are substantially higher in molecular weight.
None of the cited references claims any specific resin features based on composition, aromatic/aliphatic balance and softening point level to reach the right adhesion performances as described and claimed in the present invention. There is no relevance to be found in them according to the solution that the present invention has developed.